Can dark chocolate help control blood pressure?

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Behind the Headlines

Thursday August 16 2012

Chocolate is high in fat and should be eaten in moderation

"Chocolate … could reduce blood pressure," BBC News reports.

The report is based on a well conducted review that has pooled the results of trials investigating the effects of chemicals called flavanols. Flavanols are found in cocoa products, such as cocoa powder, dark chocolate and, to a lesser extent, milk chocolate. They are thought to widen blood vessels, causing a drop in blood pressure.

While the researchers did find a statistically significant reduction in blood pressure, the average reduction was relatively modest – a drop of 2-3mmHg.

It is not possible to say whether this small difference could have a positive effect on health or reduce risk of cardiovascular events, such as heart attack. As the researchers point out, this small drop may be useful if other methods, such as regular exercise, are also used to reduce blood pressure.

It is also worth noting that the trials only lasted a few weeks, so it is not possible to tell what the longer term effects would be – both in terms of pros and cons. The trials also varied widely in the dose of flavanol that was given, so it is difficult to determine what the ideal dose would be.

Chocolate in moderation can be part of a healthy balanced diet, but it is high in fat and calories. If eaten in excess any possible beneficial effects are likely to be outweighed by the risk of obesity, which itself increases the chance of high blood pressure and heart disease.

The Kuna Indians

The link between blood pressure and cocoa consumption was first identified in 1944 by researchers who were looking at levels of blood pressure in Kuna Indians – a tribe living on an island off the coast of Panama.

They found that tribe members who continued to live on the island, where it was (and still is) normal to drink up to 3-4 cups of cocoa a day, had lower levels of blood pressure compared with those who had migrated to mainland Panama, where cocoa drinking is much less common.

Where did the story come from?

This review was authored by members of The Cochrane Collaboration, an international, independent, not-for-profit organisation that produces systematic reviews. The current review was supported by The University of Adelaide, Australia, and the Australian Government Primary Health Care Research Evaluation Development (PHCRED) programme. The individual trials included in the review received funding from various sources, which included in some cases cocoa industries and companies. The authors of the review took into account the potential bias of funding sources into their analyses.

BBC News gives accurate and balanced coverage of this research and they did stress that “there are healthier ways of lowering blood pressure”.

What kind of research was this?

This was a systematic review and meta-analysis that aimed to identify all trials investigating the effects of chocolate or cocoa products on blood pressure, and then pool their results. The authors say that the flavanol chemicals that are found in cocoa stimulate nitric oxide, which causes dilation (widening) of blood vessels, and so may be linked to lowering blood pressure. In general, the darker the chocolate is the more flavanols it contains, so the researchers were examining products known to be high in flavanol, such as dark chocolate.

The possible cardiovascular effects of chocolate have been frequently studied in the past. A systematic review published last year looked at the results of observational studies examining the effect of chocolate consumption on the risk of cardiovascular disease. This review found some evidence of an association, but its results were limited as the studies included were observational studies and not randomised controlled trials.

A systematic review including all relevant randomised controlled trials is the best way of investigating the effect of a particular intervention (in this case, cocoa or chocolate) upon an outcome (in this case, blood pressure). Systematic reviews may have limitations if the trials they include have different designs and methods, such as differences in study population, intervention dose and comparator, trial duration, and measurement of outcomes. When the results of the different trials vary significantly from each other as a result, this is known as heterogeneity.

What did the research involve?

The authors searched relevant electronic medical databases to identify all randomised controlled trials that were of at least two weeks’ duration and had compared the effects of chocolate or cocoa products on blood pressure with a control product. The control could be either a flavanol-free or low-flavanol product, but if the control did contain flavanols they had to be less than 10% of the dose in the chocolate or cocoa being tested. The trials could include adults either with or without high blood pressure (hypertension).

The main outcome of interest was the difference in systolic (the upper figure of the two-figure blood pressure measurement, for example 120 in 120/80) and diastolic blood pressure (the lower of the two figures) at final follow-up between cocoa and control group. Other outcomes of interest included compliance with treatment, and adverse effects or intolerance of treatment.

Researchers assessed the quality of the trials and took into account any bias that might influence study results, and any difference in study results. They pooled results of all trials looking at the effects of cocoa or chocolate upon blood pressure, and also separate analyses for trials that used a flavanol-free control group, and those that used low-flavanol controls.

What were the basic results?

The researchers identified 20 relevant studies, which included 856 mainly healthy adults. Trial duration varied between two and 18 weeks, the average duration being 4.4 weeks. The daily flavanol dose in the intervention group ranged between 30 and 1,080mg, the average dose being 545.5mg of flavanols contained in between 3.6 and 105g of cocoa products. In 12 trials the control group was given a flavanol-free product, and in the remaining eight trials the control was cocoa powder that contained a low dose of flavanols (between 6.4 and 41mg).

Pooled results of all trials revealed a small, but statistically significant, greater reduction in blood pressure with flavanol-rich cocoa products compared with control:

a 2.77mmHg greater reduction in systolic BP in the intervention group compared with control (95% confidence interval of the difference between the two groups 4.72 to 0.82mmHg)

a 2.20mmHg greater reduction in diastolic BP in the intervention group compared with control (95% confidence interval of the difference between the two groups 3.46 to 0.93mmHg)

Analyses restricted to those trials where the control was a flavanol-free product still observed a significant difference in blood pressure between the intervention and control groups. However, those trials that had compared a high-dose flavanol product with a low-flavanol control found no significant difference between the two groups.

The researchers found that in the nine short-term trials (only two weeks’ duration) there was a significant blood pressure difference between groups. However, in 11 trials of greater than two weeks’ duration there was no significant difference in blood pressure between groups. The researchers noted that the significant difference in the two-week trials may have been due to the fact that seven out of these nine trials had a flavanol-free control group.

Adverse effects, including digestive complaints and distaste in the mouth, were reported by 5% of participants in the cocoa intervention groups compared with 1% of participants in the control groups.

How did the researchers interpret the results?

The researchers conclude that “flavanol-rich chocolate and cocoa products may have a small but statistically significant effect in lowering blood pressure by 2-3mmHg in the short term”. They do, however, acknowledge that differences between the design and results of the studies limit the ability to draw any firm conclusions.

Conclusion

This was a well conducted systematic review that combined the results of all trials that have investigated whether cocoa or flavanol-rich chocolate have an effect on blood pressure in predominantly healthy adults. The researchers did find a small but statistically significant 2-3mmHg difference in blood pressure between the intervention and control groups. However, there are important points to be aware of, including:

Trials were of short duration

All of the trials were of short duration, the average being four weeks. The researchers say that they were unable to identify any randomised controlled trials that tested the effect of longer-term daily ingestion of cocoa products. Also of note is that the analyses that were restricted to the trials that were two weeks or longer found no significant difference in blood pressure between the groups. Therefore, we do not have any evidence on the longer-term effects on blood pressure or whether there may be unidentified side effects linked to the long-term consumption of flavanol-rich chocolates or cocoa.

Clinical relevance of the outcomes

None of the trials examined clinical outcomes related to high blood pressure, such as heart disease or strokes. It is, therefore, not possible to say whether the small 2-3mmHg difference in blood pressure measure after the trial would have actually made any difference to the health of the person or influence their cardiovascular risk.

Uncertain ideal dose of flavanol

The trials varied widely in the dose of flavanol or cocoa that was used. In trials comparing high-flavanol with low-flavanol products no blood pressure difference was found, only in those trials comparing high-flavanol with flavanol-free controls. From this it is not possible to say what the ideal dose of flavanol would be and, as the researchers say, trials comparing low-flavanol with flavanol-free products would be valuable to see whether a lower dose has an effect on blood pressure.

Limited populations studied

The authors also say that, although they did analyses looking at participants of different age, body mass index or starting blood pressure, they do not have firm evidence of what the pressure effects would be in different population groups, and this would require assessment in further trials.

Risk-benefit balance

Chocolate in moderation can be part of a healthy balanced diet, but it is high in fat and calories. If eaten in excess any possible beneficial effect upon blood pressure is likely to be outweighed by the risk of becoming overweight or obese, which increases risk of cardiovascular disease and many other chronic diseases.

There are far more effective and healthier ways to reduce blood pressure, such as: